No. I.] SPHYRAISrURA OSLERL II 



hooks exceed the others in size and functional importance, those 

 of Sphyranura resembling the figures given by Wagener for 

 some species of Dactylogyrus more closely than do the hooks 

 of the other genera. Each hook is formed of a splint-like shaft, 

 striated on both of its flat surfaces for the attachment of one of 

 the three muscles to be afterwards described (<r, Fig. 4) ; the shaft 

 becomes rounded and stronger as it passes into the curved part 

 of the hook, and at the junction of its flattened and rounded 

 parts is a strong muscular process, with protuberances for the 

 two remaining muscles attached here {a and b in Fig. 4). The 

 curvature of the hook is not only sharper than in the other 

 genera, but at the beginning of the curvature there are two 

 barbs, one on the surface of the other, which no doubt increase 

 the efficiency of the sharp point. Wagener has represented in 

 several of his figures ^ a transverse chitinous piece, which joins 

 the muscular processes of the two large hooks. This answers in 

 position to the transverse muscle {b, Fig. 4) in Sphyranura, the 

 ends of the fibre of which seem, in certain instances, to have 

 undergone a change in consistency. In specimens flattened 

 under a cover the position of the large hooks varies much 

 according to which of the three pairs of muscles has contracted 

 most violently. 



The booklets arc situated seven on each side of the caudal 

 lamina, as indicated in Fig. 4, and one in the centre of each 

 sucker ; all of them have their shafts turned towards the middle 

 line of the lamina. They resemble the large hooks in miniature, 

 having a shaft and muscular process just like these ; but at the 

 point of projection through the investing membrane each is 

 surrounded by a chitinous " eye," in all respects similar to that 

 figured by Zeller for Polystomum.^ The "eye" is thinner at 

 one spot (Fig. 5), so that it assumes an oval or circular outline, 

 according as it is or is not pressed against by the booklet which 

 projects through it. Slender muscular bundles pass to the 

 shaft and muscular process of the booklets ; but these hardly 

 attract attention, except in the case of the acetabular booklets, 

 where they have to perforate the centre of the sucker to reach 

 their attachment {m, Fig. 3). 



' Beitrage zur Entwickelungsgeschichte der Eingeweidewlirmer ; Haarlem, 1857, PI. 

 XV., Fig. 5. 



2 Zeit. wiss. Zool., XXII., Tab. II., Fig. 2. 



